A GPL'd compiler is an issue for various projects but especially since GCC moved to GPLv3.

I don't know why you've been marked down given that Apple's reason for supporting LLVM/Clang was so that they can heavily integrate XCode and the compiler together so that meaningful error messages can be provided to the programmer as to debug their programme. There are many other considerations but it is amazing how something that seems like common sense is being turned by licence zealots into a religious cause.